I. Field of the Invention
The present invention relates to the field of selective plating, i.e., plating selectively only the contact surfaces of connector parts to the exclusion of other surfaces of the parts.
II. Background of the Related Art
Metal plating is the process of electrolytically depositing a layer of metal onto a part. Engineers and manufacturers often use precious metals, such as gold, silver, platinum, palladium or rhodium, for plating because of their exceptional resistance to corrosion. Precious metal plating also has low contact and electrical resistance making it ideal for electronic applications. Many electronic parts, such as connectors or circuit board components, are plated with precious metals to provide maximum corrosion resistance while maintaining excellent conductivity and solderability.
Generally, coating of many metals can be deposited on other metals, and on non-metals by electroplating (also referred to as electrodeposition), when suitably prepared. This is based on the principle that when direct current power of high enough voltage is applied to two electrodes immersed in a water solution of metallic salt, current will flow through the circuit causing changes at the electrodes. At the negative electrode, or cathode, excess electrons supplied from the power source neutralize positively charged metallic ions in the salt solution to cause dissolved metal to be deposited in the solid state. At the positive electrode, or anode (plating metal), metal goes into solution to replace that removed at the other electrode. For example, if the anode is made of nickel and the cathode is a copper part, then when a current is applied, positively charged ions (small pieces of metal) from the anode flow through the solution to the cathode and attach themselves to the part, producing a layer of nickel on the part. The rate of deposition and the properties of the plated material are dependent on the metals being worked with, the current density, the solution temperature, and other factors. The baths that are used for plating vary from acid to neutral to alkaline with many different chemical formulations involved. Phosphates, sulfates and carbonates, usually of the plating metal, are also commonly added to the electroplating bath. These plating chemicals help to increase and maintain the electric conductivity of the solution.
Controlling the thickness of the electroplated part is generally achieved by altering the time the object spends in the salt solution. The longer it remains inside the bath, the thicker the electroplated shell becomes. The shape of the part will also have an effect on the thickness. Sharp corners will be plated thicker than recessed areas. This is due to the electric current in the bath and how it flows more densely around corners. Before electroplating a part, it must be cleaned thoroughly and all blemishes and scratches should be polished. As described above, recessed areas will plate less than sharp corners, so a scratch will become more prominent, rather than being smoothed over by the plated material.
Another method of plating involves electroless plating which is similar in result to electroplating in that a metallic layer is deposited onto the surface of a part. Electroless plating, however, uses a chemical deposition process—instead of an external electrical current—to achieve the desired result. Electroless plating generally is used for nickel plating, following which a standard chrome plate can be applied to the plastic workpiece in a conventional electroplating bath. Chrome plating plastic offers enhanced protection from corrosion and weather, and plastic or other non-conductive surfaces like fiberglass appear virtually identical to metal workpieces that have been electroplated with chrome.
Many elemental metals and some selected alloys can be used as plating materials. Each material has certain benefits that make it applicable to specific applications. For example, gold is an excellent conductor and resists corrosion caused by formation of oxides that reduce conductivity, so it is often used for electrical contacts and connectors. Other commonly deposited materials include silver, copper, nickel, tin, solder, brass, cadmium, palladium, zinc, etc. Generally, the precious and semi-precious metals are the preferred choice for enhanced conductivity of the connector parts. The high cost of these metals, however, has necessitated precision deposition on the electrical contact surfaces of the parts, while specifically excluding the surfaces of the parts on which plating is unnecessary.
For certain bath and base material combinations it is necessary to create an adherent layer to improve the electroplating process. A strike or a flash is a preliminary electroplating step that applies a thin, but highly adherent layer on the base. A strike serves as a foundation for subsequent plating processes. It uses a high current density and a bath with a low ion concentration. The process is slow, so more efficient plating processes are used once the desired strike thickness is obtained. The striking method is used in combination with the plating of different metals. If it is desirable to plate one type of deposit onto a metal to improve corrosion resistance, but this metal has inherently poor adhesion to the substrate, a strike can be first deposited that is compatible with both. One example of this situation is the poor adhesion of electrolytic nickel on zinc alloys, in which case a copper strike is used, which has good adherence to both.
In the case of gold plating of electrical connectors, e.g., gold-on-copper plating, the copper atoms have the tendency to diffuse through the gold layer, causing tarnishing of its surface and formation of an oxide/sulfide layer. Thus, a layer of a suitable barrier metal, usually a nickel strike, has to be deposited on the copper substrate, forming a copper-nickel-gold sandwich.
Previous prior art plating apparatuses in order to prevent deposition of plating onto the entire part, involved a step of masking the portion of the part which did not require plating by covering it with plugs, caps, resists, or lacquers which are then removed after plating. Masking, however, required another manufacturer operation. Some immersed surfaces are also difficult to mask, particularly the surfaces of small size electrical terminals of various connector pins and sockets. The present invention accomplishes, among other things, selective plating according to an automatic process without a need for masking immersed part surfaces on which plating is unnecessary.
Another prior art solution is to plate the entire part due to its small size irrespective of the actual electrical contact surface. While such solution offers certain simplicity, it greatly increases the amount of plating metal used to cover the entire part. Due to the constant increase in the cost of gold and other precious metals, it has become highly desirable to plate only the selective contact terminals of the part. The present invention reduces the amount of plating metal utilized by more than 80% by selectively plating only the necessary electrical contact terminals of the part.
More recent approaches to selective electroplating solutions have concentrated on building selective plating equipment custom tooled for pin or socket type connectors of particular shape and size. The new selective plating technologies provide the ability to plate only the most critical areas of the part. This ability to precisely control the deposit saves a considerable amount of money by reducing the amount of gold used during the process, while simultaneously enhancing part performance in terms of wear resistance, corrosion resistance, conductivity and solderability. Generally, these modern selective gold plating machines use vibratory or barrel plating using Sulfamate Nickel, followed by a selective gold strike. These prepared parts are then transferred to an automatic or manually loaded selective plating equipment tooled for pin or socket type connectors as shown in FIG. 1 where gold is deposited only where it matters most with regard to performance. Alternatively, selective gold plating of connector pins or sockets can be directly deposited onto Nickel plating without the gold strike.
The problem with the more modern approaches to selective electroplating, however, is the equipment's inability to handle parts of various shapes and sizes. For every new type of connector pin or socket a completely new custom tooled plating machine must be built. This is especially true for the connector parts where the center of gravity is at different locations depending on the shape of the connector part, resulting in the connector part, such as a pin, always turning in one direction which causes the selective plating to be applied on the opposite and the wrong end of the connector. FIG. 1 illustrates different types of pins to be plated with the flanges (shoulders) located at different positions along the length of the pin, thus resulting in different centers of gravity position for each. With the constant improvements in the electronic industry the connector parts never stay the same. The cost of such custom machines is high, thus making the upgrade path cost-prohibitive for the electronics manufacturer, thereby stalling the technological improvements in the electronic equipment. Accordingly, there is an increasing demand for the selective electroplating equipment to handle parts of any shape or size irrespective of their center of gravity.
For example, the EP 0070694, entitled “Conveyor Apparatus For Use In Electroplating And An Electroplating Machine,” assigned to Kirkby Process & Equipment Limited, which is incorporated herein by reference in its entirety, discloses a conveyor apparatus which enables the electroplating process to be applied only to a part of a component, leaving the rest uncoated. The conveyor apparatus comprises a rotatable carrier and an endless band which passes around at least part of the circumference of the carrier to hold parts for plating in electrical contact with the carrier. The rotatable carrier is a one-piece metallic wheel whose circumference has a plurality of equi-spaced notches for receiving the parts. The parts to be held by the band pass along a radially disposed stationary channel under the influence of a vibratory feed device. Each part in turn contacts the carrier and is picked up and carried along by one of the notches passing the feed device. A guide disposed immediately adjacent the feed device maintains the articles in contact with the carrier and in the correct axial location until the parts are in a position to be contacted and thereby gripped by the band. Notably, the guide has a shoulder with which the parts engage prior to being held by the band and this should ensure that successive parts are all similarly positioned axially with respect to the carrier wheel. The problem with this approach is that parts are always positioned and turned in the same exact orientation based exclusively on their center of gravity. Therefore, the parts will always be oriented in such a way as to apply the plating onto the heavier end of the part. Such prior art solution leads to plating of the wrong contact surface of differently shaped parts with different centers of gravity. Moreover, since the parts are always turned in only one direction, plating is limited to only one side of the part.
Another disadvantage of the above-mentioned approach is the carrier's handling of the parts exclusively via the physical notches or grooves on the carrier. While the parts may fit perfectly within the precision tooled grooves, such constant physical friction between the grooves and the parts causes unnecessary stress on the parts resulting in their damage or scratches which lead to uneven plating.
The present invention is characterized in that connector parts of various shapes and sizes can be automatically and continuously fed to the electroplating equipment without unnecessary physical stress on the parts. The electroplating apparatus of the present invention efficiently deposits plating metal only onto the necessary electrical point of contact of the parts irrespective of their center of gravity. Thus, either side of the part may be plated.